[Docs] Migrate from Sphinx to MkDocs

[rafvasq]

• Migrates to MkDocs to keep in sync with vLLM (Migrate docs from Sphinx to MkDocs vllm#18145)
• Brings spyre overview to the home page
• Updates requirements-doc.txt
• Updates admonitions and code snippets
• Adds doc development instructions

[github-actions]

👋 Hi! Thank you for contributing to vLLM support on Spyre.
Just a reminder: Make sure that your code passes all the linting checks, otherwise your PR won't be able to be merged. To do so, first install the linting requirements, then run format.sh and commit the changes. This can be done with uv directly:

uv sync --frozen --group lint --active --inexact

Or this can be done with pip:

uv pip compile --group lint > requirements-lint.txt
pip install -r requirements-lint.txt
bash format.sh

Now you are good to go 🚀

[prashantgupta24]

I like this way better!

[prashantgupta24]

do we still want these 2 commented lines?

[rafvasq]

Removed ✅

[prashantgupta24]

LGTM! Some questions/nits